Biosensors are not new. Their use in the determination of concentrations of various analytes in fluids is also known.
Nankai et al., WO 86/07632, published Dec. 31, 1986, discloses an amperometric biosensor system in which a fluid containing glucose is contacted with glucose oxidase and potassium ferricyanide. The glucose is oxidized and the ferricyanide is reduced to ferrocyanide. (This reaction is catalyzed by glucose oxidase.) After two minutes, an electrical potential is applied and a current caused by the re-oxidation of the ferrocyanide to ferricyanide is obtained. The current value, obtained a few seconds after the potential is applied, correlates to the concentration of glucose in the fluid.
Because Nankai et al. discloses a method in which the reaction of glucose and ferricyanide may run to completion prior to the application of an electrical potential, this method is referred to as the "end-point" method of amperometric determination.
Nankai et al. discloses a system, wherein the glucose oxidase and potassium ferricyanide are held on a non-woven nylon mesh. The mesh is positioned so that it is in contact with a working electrode, a counter electrode and a reference electrode. The total surface area of the counter and reference electrodes is twice that of the working electrode.
Wogoman, EP 0 206 218, published Dec. 30, 1986, discloses a biosensor having two electrodes, the electrodes being made of different electrically conducting materials. For example, the anode is formed from an anode material, such as platinum, and the cathode is formed from a cathode material, such as silver. The anode is coated with an enzyme. In a preferred embodiment, the coated electrode is covered with an elastomer that is permeable to glucose.
Pottgen et al., WO 89/08713, published Sep. 21, 1989, discloses the use of a two electrode biosensor, wherein the electrodes are made of the same noble metal, but one of the electrodes (referred to as a pseudo-reference electrode) is larger than the other (working) electrode.
Conventional wisdom in the electrochemical arts suggests biosensors of the following types:
1) a three electrode system, wherein a working electrode is referenced against a reference electrode (such as silver/silver chloride) and a counter electrode provides a means for current flow; PA1 2) a two electrode system, wherein the working and counter electrodes are made of different electrically conducting materials; and PA1 3) a two electrode system, wherein the working and counter electrodes are made of the same electrically conducting materials, but the counter electrode is larger than the working electrode.
Conventional wisdom in the electrochemical arts does not suggest that a biosensor could include a two electrode system, wherein the working and counter electrodes are substantially the same size (or wherein the counter electrode is smaller than the working electrode) and made of the same electrically conducting material.